Picosecond and femtosecond optical pulses are finding wide application in diverse technologies such as communications and electrooptic sampling. Modelocking is generally employed to obtain short pulses from the laser. Nonlinear external cavities coupled to a solid state laser have permitted generation of much shorter pulses than those available from the solid state laser itself. This phenomenon is understood to occur because the nonlinear external cavity increases the modelocked bandwidth of the solid state laser and permits more efficient communication of phase information between modes coupled in the cavity.
One striking example of a modelocked laser having a nonlinear external cavity is the soliton laser described by Mollenauer in U.S. Pat. No. 4,635,263 and in Optics Letters, Vol. 4, No. 1, pp. 13-15 (1984). The soliton laser includes a main cavity coupled to a nonlinear external cavity. The main cavity comprises a gain medium such as a solid state element which is optically pumped by another laser. Pumping is either continuous wave or pulsed. For the nonlinear external cavity, coupling to the main cavity requires that the phase of the signal injected back into the main cavity depends on intensity such as by a nonresonant Kerr effect nonlinearity. When employing an optical fiber to provide the nonlinear effect in the external cavity, this laser supports soliton pulse formation when the net group velocity dispersion is negative, that is, when group delay decreases with increasing frequency.
While the soliton laser uses the negative group velocity dispersion of the fiber in the coupled nonlinear external cavity, other important lasers have been developed using positive group velocity dispersion of the fiber. Although the lasers do not form soliton pulses, short pulse formation has been observed. Additive pulse modelocked lasers (APM) and coupled cavity modelocked lasers (CCM) are examples of such lasers wherein the nonlinear external cavity is matched to the main cavity in such a way that the returning pulse adds coherently with the main cavity pulse. As a result, chirp is caused by the nonlinear external cavity causes constructive interference at the peak of the optical pulse while simultaneously causing destructive interference in the wings or extremities of the optical pulse. This facilitates the pulse shaping and shortening process. Additive pulse modelocked lasers have been described by Ippen et al., in J. Opt. Soc. Am. B, Vol. 6, No. 9, pp. 1736-45 (1989) and coupled cavity modelocked lasers have been described by Kean et al. in Optics Letters, Vol. 14, No. 1, pp. 39-41 (1989).
While pulses as short as several hundred femtoseconds have been reported with some of the lasers described above, certain technical problems hinder development and progress with these lasers. Difficulties arise with respect to realizing a working arrangement of the necessary laser elements. Typical realization times for additive pulse modelocked lasers are on the order of several months to, in some cases, one year. Alignment of the elements tends to be a major contributing cause. Coupling from one cavity to the other is further complicated when the external cavity employs a waveguide element such as an optical fiber. Moreover, for lasers having optical fiber in the external cavity, nonlinear effects are increased solely by increasing the length of the optical fiber. Finally, it is important to note that, in practice, most of the lasers described above require some external stimulus such as a narrow noise peak to begin pulse generation.